It is conventional to use polymethylmethacrylate (PMMA) in the manufacture of molding materials which make plastic mold parts. The resulting molded articles often exhibit a high transparency and an excellent optical quality. Unfortunately, the usefulness of PMMA molding materials is limited by the softening properties of PMMA: at temperatures above approximately 115.degree. C. PMMA softens and is no longer useful. Furthermore, the "service temperature" (i.e., the highest temperature at which PMMA can be stored for any reasonable length of time) is even lower than the softening temperature, such that at temperatures &gt;95.degree. C. PMMA exhibits deficient storage life and serviceability.
Consequently, at higher temperatures other plastics must be used. For example, polycarbonates having a Vicat temperature of approximately 150.degree. C. can be considered as transparent plastics for this purpose. However, like PMMA, the service temperature of polycarbonates is far less than the Vicat temperature--about 20.degree. C. lower (or approximately 130.degree. C.). Compared to PMMA, polycarbonates are very sensitive to scratches and are significantly less resistant to atmospheric corrosion.
Other transparent plastics having a high thermal dimensional stability include polymethacrylimides, which have a Vicat temperature of up to 175.degree. C. at complete imidization. Polymethacrylimides are obtained by reacting methacrylic polymerizates with a primary amine in a reactor. The imide structures form on the side arms of the PMMA macromolecule. The Vicat temperature rises with the degree of imidization.
Polymethacrylimides have greater water absorption properties than PMMA. The production and properties of this polymer are known from the patent DE-A-40 02 904.
DE-A-26 52 118 describes a method for imidization of polymerizates of acrylic- and/or methacrylic acid esters, novel, imidized thermoplastic polymerizates and a molding material of such polymerizates. Such polymethacrylimides with Vicat temperatures of between 134.degree.-163.degree. C. are commercially available.
It is known that the thermal dimensional stability of PMMA can be increased by copolymerization of methylmethacrylate (MMA) with suitable monomers. For example, MMA can be copolymerized with styrene and maleic acid anhydride. French patent 1,476,215 describes copolymers of MMA with styrene and maleic acid anhydride, wherein the copolymers have a Vicat temperature between 130.degree. and 145.degree. C. However, due to the aromaticity of the copolymer, the resulting plastic does not resist atmospheric corrosion as well as plastics formed with PMMA.
British patent 641,310 describes the mass polymerization of .alpha.-methylene-.gamma.-butyrolactone and .alpha.-methylene-.gamma.-methyl-.gamma.-butyrolactone. Dibenzoyl peroxide, azo-bis-isobutyronitrile or radiation with a mercury lamp are described as initiators for the radical polymerization. Similarly, U.S. Pat. No. 2,624,723 describes homopolymers of .alpha.-methylene-.gamma.-butyrolactone and .alpha.-methylene-.gamma.-methyl-.gamma.-butyrolactone. This patent also describes a copolymer of .alpha.-methylene-.gamma.-butyrolactone with acrylonitrile. The polymers are produced by radical polymerization with peroxides as initiators, or by irradiation with UV light. The compounds in these two patents are characterized by high glass transition temperatures. They are hard and brittle and have a slightly yellow color. In addition, these polymers and copolymers are produced without chain transfer agents, and consequently have a relatively high molecular weight which makes them unsuitable as molding materials. Indeed, even their usefulness for producing cast glasses is doubtful.
Macromolecules 12, pp. 546-551 describes the homopolymer of .alpha.-methylene-.gamma.-butyrolactone and its glass transition temperature (Tg). In this reference, a relatively high Tg of 195.degree. C. and low solubility are indicative of considerable stiffness of the polymer chain.
Furthermore, copolymerization studies of .alpha.-methylene-.gamma.-butyrolactone and methylmethacrylate (Polymer 20, pp. 1215-1216) showed that the former disposes over a distinctly higher reactivity than the latter.
Dent Mater 8, pp. 270-273 describes the use of exo-methylene lactones as a comonomer in dental resins. It was found that exo-methylene lactones further the conversion of dental resins in the polymerization by lowering the viscosity. The result is a harder dental filling.
More specifically, the production of homopolymerizates from .alpha.-methylene-.gamma.-butyrolactone, .alpha.-methylene-.gamma.-phenyl-.gamma.-butyrolactone, 3-methylene-1-oxaspiro4.5!decan-2-one as well as methylene phthalide is known from this reference. Data for synthesis for the three latter exo-methylene lactones is also presented. The polymerization of .alpha.-methylene-.gamma.-butyrolactone in a binary mixture with bis-GMA (2,2-bis-4-(2-hydroxy-3-methacryloxypropoxy)phenylene! propane) or in a ternary mixture with bis-GMA and TEGDMA (triethylene glycol dimethacrylate) to obtain the corresponding copolymers is known.
According to the Dent Mater reference, the homopolymerizates of the exomethylene lactones produced are glassy, brittle materials. In contrast, the use of .alpha.-methylene-.gamma.-butyrolactone with the bifunctional methacrylates acting as cross-linking agents results in copolymers which exhibit a higher degree of cross-linking and, consequently, an increased conversion which leads to a greater hardness. Although the hardness of the copolymers was increased as a result of increased conversion, the copolymers exhibit a thermal dimensional stability which was unsatisfactory.
Furthermore, the radical copolymerization of .alpha.-methylene-.gamma.-butyrolactone with styrene in various ratios is known from the Journal of Polymer Science: Polymer Chemistry Edition, vol. 20, 00. 2819-2828 (1982). The experiments described in this reference served to determine the parameters of copolymerization. As the results indicate, .alpha.-methylene-.gamma.-butyrolactone is very reactive as a cyclic analogue to MMA and has greater Q and e values than the MMA. The reference also mentions that the homopolymer of .alpha.-methylene-.gamma.-butyrolactone is more thermally stable than the polymethylmethacrylate. According to the reference, thermal stability is understood as resistance to depolymerization or thermal decomposition. Thus, the differential thermoanalysis of the .alpha.-methylene-.gamma.-butyrolactone homopolymerizate showed an endothermy at approximately 320.degree. C., whereas that of PMMA was approximately 50.degree. C. lower.
Finally, Makromol. Chem. Rapid Comm. vol. 3, pp. 311-315 (1982) describes the alternating ionic polymerization of .alpha.-methylene-.gamma.-butyrolactone with styrene in the presence of a Lewis acid. Macromolecules 1986, 19, pp. 558-565, describes the radical copolymerization of .alpha.-methylene-.delta.-valerolactone with styrene. Methods of producing the homopolymer of .alpha.-methylene-.gamma.-methyl-.gamma.-butyrolactone by means of radical, anionic or group-transfer polymerization are accessible from Macromolecules 1984, 17, pp. 2913-2916.